This invention relates generally to liquid ink-jet printers and more particularly to the use of high surface tension slow-drying ink which is dried in a manner to maintain high image quality. While not limited to, the present invention finds particular benefits when used in conjunction with acoustic ink printing.
Acoustic ink printing is a potentially important direct marking technology. It compares favorably with conventional ink-jet systems for printing either on plain paper or on specialized recording media while providing significant advantages of its own.
Drop-on-demand and continuous-stream ink-jet printing systems have experienced reliability problems because of their reliance on nozzles with small ink ejection orifices which easily clog and which limit the life as the size of an ejected ink droplet is decreased. Acoustic printing obviates the need for such nozzles, so it not only has a greater intrinsic reliability than an ordinary ink-jet printing system, but also is compatible with a wider variety of inks, including inks which have relatively high viscosities and inks which contain pigments and other particulate components. Furthermore, it has been found that acoustic printing provides relatively precise positioning of the individual printed picture elements (xe2x80x9cpixelsxe2x80x9d), while permitting the size of those pixels to be adjusted during operation, either by controlling the size of the individual droplets of ink that are ejected or by regulating the number of ink droplets that are used to form the individual pixels of the printed image. See a co-pending and commonly assigned U.S. Patent Application of Elrod et al. which was filed Dec. 9, 1986 under Ser. No. 944,286 on xe2x80x9cVariable Spot-Size Acoustic Printingxe2x80x9d, hereby incorporated by reference.
When an acoustic beam impinges on a free surface (i.e., liquid/air interface) a pool of liquid from beneath the radiation pressure which the beam exerts against the surface of the pool reaches a sufficiently high level to release individual droplets of liquid from the pool, despite the restraining force of the surface tension. Focusing the beam on or near the surface of the pool intensifying the radiation pressure it exerts for a given amount of input power. The basic principles of acoustic-ink printing are well known and the subject of numerous commonly assigned U.S. patents.
A specific benefit of acoustic-ink printing is the ability to generate droplets which are of a much smaller size than the orifice through which the droplets are ejected. It has been found that acoustic-ink printing can generate droplets which are a magnitude smaller in size than that of the orifice opening, and significantly smaller than existing conventional ink-jet printer systems. This allows an acoustic-ink printing system to generate high resolution images not previously obtainable, since a key factor in obtaining high resolution is depositing the smallest spot possible on a recording medium.
However, in existing printing methods, both for conventional ink-jet printing and acoustic ink printing, the present practice is to use fast penetrating inks (also known as fast drying or low surface tension inks) for aqueous ink-jet printing. The fast penetrating inks are those which will commonly penetrate into plain paper fiber in less than three seconds allowing the ink to spread quickly on the surface of the paper and also seep into the paper.
A benefit of using fast drying inks is in conjunction with color printers, in order to reduce inter-color bleeding which would commonly occur if using slow drying inks, also known as high surface tension inks.
Another benefit of using fast drying inks for color printing is that as the inks are laid down on the print medium (e.g. paper such as plain paper), when a second color ink is placed down on that location or adjacent thereto, the first laid down ink will not tend to be on the surface, i.e. it will already have been absorbed into the paper. Therefore, the second laid down ink will not run over the first ink. The fast penetrating ink wicks into the paper before the second color ink is jetted onto the same paper surface. Additionally, the penetration of the first ink is rapid enough that lateral migration into adjacent locations previously printed is reduced, thereby diminishing inter-color bleed, which would normally occur under conventional techniques of printing with slow drying inks.
However, there are several drawbacks to use of fast-drying inks. Particularly, by having the ink penetrate into the paper some portion of the colorant or dye is also transported into the paper. This results in low optical density of the printed materials and also greater show-through when viewing the paper from the non-printed side. Specifically, the more colorant which is moved into the paper lowers the amount of colorant which can be visualized by a viewer, since the fibers will block the colorant from view.
Existing conventional ink jet printing machines which use fast drying inks can expect to obtain 1.2 to 1.3 optical density, when using plain paper. This is compared to high quality xerography at 1.8 to 2.0 and photography at 2.1 to 2.3 optical density.
A drawback of backside show-through is the inability to do duplex printing. Particularly, since the use of fast drying ink will, in many cases, cause the ink to wick through to the opposite side of the paper, two-sided printing would not be possible, since the ink which shows-through to the opposite side would ruin the second print.
The fast penetration/wicking characteristic of fast-drying ink into the paper also has the effect of some lateral wicking depending on the surface topology of the paper. This causes a poor edge sharpness on printed lines and text.
As discussed in U.S. Pat. No. 5,771,054 to Dudek et al., commonly assigned and hereby incorporated by reference, high-edge-sharpness is desirable in any printer. The typical goal is a laser-quality print. Color printers typically focus on the quality of the color reproduction and have less concern for edge definition. Black ink-jet printers that can yield sharp edges on plain paper are inherently slow-drying. This means that a page will still be wet and smudgable when output unless substantial amounts of drying time and/or thermally assisted drying are provided. Acoustic ink printing is desirable for its ability to provide edge-sharpness, without ragged edges, since it can apply such small drops which allow for a high dots-per-inch value.
When color printing, inter-color bleed is reduced by the use of fast-drying inks. While fast-drying inks have lower edge definition, in existing systems, they are still used for color reproduction. Also for existing systems, a color printer might use a slow-drying ink for monochrome black text and graphics, and use fast-drying color inks for color reproduction. Under this use, it is common that the slow-drying of the black ink causes inter-color bleed when used with color inks in normal printing or it will require substantial drying time.
A key aspect of printing is to remove the liquid from the ink droplets deposited on the recording medium. For example, liquid can be removed from the ink and printed medium by a number of methods. One simple method is natural air-drying in which the liquid component of the ink deposited on the medium is allowed to evaporate without mechanical assistance resulting in natural drying. Another method is to send the printed substrate through a dryer to evaporate the liquid. In some cases a special paper is used in which the liquid is absorbed by a thin coating of absorptive material deposited on the surface of the paper. Blotting of the printed medium is also known.
In the case of natural drying, almost 100% of the liquid is absorbed into the paper and is then, over a long period of time, evaporated naturally. The absorption and de-absorption of water into and out of the paper, however, has some undesirable side effects, such as a long drying time, strike through, feathering at edges of the printed image, paper curl and paper cockle. In the case of paper cockle, the absorption and de-absorption of the water relaxes the internal stresses of the paper, resulting in cockle. Cockle is also a function of the amount of liquid deposited per liquid area. Less printing on a paper has less potential to develop cockle due to the small amount of liquid. More printing on a paper has more cockle potential due to a higher amount of liquid per unit area. Cockle can also be induced by heating the paper, which results in stress relief.
Various drying mechanisms for drying images deposited on recording mediums are illustrated and described in the following patents which may be relevant to certain aspects of the present invention are hereby incorporated by reference, including U.S. Pat. Nos. 5,742,315, 5,231,426, 5,754,208, 5,757,407, 5,631,685, 5,771,054, and 4,751,529. The concept of drying disclosed in these patents is primarily directed to applying heat from the backside of the paper being printed on and generally to perform the drying after completion of the printing process.
It is therefore an object of the present invention to provide a printing and drying method and apparatus which enables high quality aqueous ink-jet printing on a recording medium at high speeds. The aspects of high quality that this method and apparatus enables is high optical density, high edge sharpness, low inter-color bleed, low show-through, and the absence of paper cockle. The high quality printing is obtained in part by maintaining the drop size placed on a paper area by avoiding substantial wicking of the ink laterally on and into the recording medium. The objects of the invention are achieved by use of high-surface tension (slow drying) inks in a manner considered inappropriate by conventional standards.
In accordance with one aspect of the present invention there is provided an ink-jet printing apparatus for printing on a recording medium such as plain paper as well as other types of paper. The ink printing apparatus jets aqueous ink drops on the paper in the form of an image where the aqueous ink is a slow penetrating ink which does not penetrate the paper/paper fibers for a relatively long time, on the order of greater than three seconds. Further provided is a drying system which allows for rapidly evaporating the water from the ink while the ink is still resident on the paper surface. The evaporation process is provided to substantially dry the initial ink before a second ink is jetted onto the paper at substantially the same, adjoining or other location. The evaporation or drying process is rapid enough to prevent the deposited ink from substantially migrating/wicking to any adjacent location of the paper which has or does not have ink laid thereon.
A primary advantage of the present invention is maintaining high-quality, high-density printing with high-edge sharpness, low inter-color bleed, low show-through, and the absence of paper cockle.